In short: if you want to hide from the HTML code "<meta name="generator" content="WordPress ..." />, as well as the exact WP version you are using, all you need to do is (in case that you don’t find the code inside header.php — for example, in WP up to version 2.3.3, the code was usually there):

1) Locate and open with a text code editor the file functions.php inside the theme directory — for example, if you are using the Default Theme, the functions.php will be in wp-content/themes/default/functions.php.
2) Add inside it the following code:remove_action('wp_head', 'wp_generator');
3) Save the file functions.php and upload it back to the server.

That’s it! Automagically, the code, which shows that you are using WordPress, as well as the exact version of it, will disappear from the HTML source! :-)